Voltage references are used to transfer the standard unit of voltage from a National Institute of Standards (“NIST”) traceable standard to a laboratory and ultimately to an instrument or measurement circuit. Decades ago, standard chemical cells served this purpose. But, standard cells typically comprise hazardous elements such as Cadmium and Mercury in glass tubes. Even when further encased in outer cases, they are still subject to slight changes in voltage with mechanical upset and temperature.
Major laboratories needing the best primary voltage references can now use a voltage standard based on Josephson junctions. When cooled to superconducting temperatures and irradiated by a precise microwave frequency, hundreds of Josephson junctions wired in series provide a standard voltage based on physical constants. Such standards however are very expensive, costing in excess of ten thousand dollars. And they need a reservoir of liquid Helium to cool the Josephson junctions. Such standards are not practical for smaller companies or laboratories. And, they are neither small enough nor cost effective for use in precision instrumentation circuits.
Temperature compensated zener diodes, including the 1N821 family, and in particular 1N829A compensated zener diodes are stable enough to fulfill many precision voltage reference applications. Since the 1N829A diode is very sensitive to zener bias current, the reference diode bias current must be provided by a precision current source or a resistor coupled to the reference output in a “bootstrap” topology. By using precision non-inverting OpAmp configurations, a variety of desired output voltages, such as 10.000 Volts can be derived from the diodes fixed voltage near 6.2 V.
Another zener based voltage reference uses a small device comprising a zener diode type semiconductor element, an internal heater, and a temperature control circuit. The internal heater of the device package maintains the internal semiconductor elements at nearly constant temperature. Such devices can further include second and third order correction devices such as junctions and transistors on the same die. This type of device is typically packaged in a standard semiconductor package. The device package can further comprise optional outer layers of insulation, such as a plastic outer casing, to further thermally isolate the tiny oven controlled environment from the ambient atmosphere. Classic devices of this type include the National Semiconductor LM199/LM399 families. More sophisticated versions, as Linear Technology's LTZ1000 families are capable of the highest levels of precision with short term changes in voltage with temperature of better than 0.1 part per million (“ppm”)/degree C. These devices however can only generate a single voltage typically falling between 6.8 and 7.1 Volts. And, they do not include an internal amplifier for driving reference loads.
Monolithic zener based voltage reference chips are the most commonly used voltage reference chips. These chips draw their precision from zener type devices and bandgap references. Monolithic voltage reference chips typically also include an internal non-inverting OpAmp circuit to generate a predetermined output voltage. Common output voltages are 1.024, 2.048, 4.096, 5.000, and 10.000 Volts. Any voltage can be generated by use of the appropriate internal resistors. One or more of these resistances are usually laser trimmed to bring absolute output voltages to within 1 to 0.1% of the rated voltage. Stability with temperature for monolithic reference integrated circuit (“IC”) chips generally ranges from 5 to 50 ppm/degree C.
What is needed is an inexpensive circuit topology that can achieve the temperature stability of ovenized zener devices while offering the choice of output voltages and output drive capability available from monolithic references.